In the process of forming thin films on a wafer, different gases must be successively introduced into a vacuum chamber where the films are deposited on the wafer. The different gases are introduced into the chamber through a gas feed piping system which must be cleared of the residual portion of one gas before another gas is introduced. In a typical prior art gas feed piping system as illustrated in FIG. 6, different gases G.sub.1, G.sub.2, G.sub.3, G.sub.4 and G.sub.5, are applied to the process chamber through a pipe 44 by selectively opening block valves 40 one at a time. Block valves permit the internal volume of the piping passages to be smaller than if single valves are used. This has the advantage of making it easier to replace gas in the piping and generation of particles can be reduced.
In changing over from a first gas to a second gas, the prior art piping system employs a vacuum pump 43 connected to the piping incorporating the block valves, the vacuum pump being connected through a branch pipe 41 and a valve 42 to a point downstream of the block valves. The changeover is accomplished by closing the block valve 40 supplying the first gas and then opening the valve 42 so that vacuum pump 43 may evacuate the residual portion of the first gas from the system. However, if the inner volume of the piping is relatively large, the degree of vacuum in branch pipe 41 drops sharply when valve 42 is opened. Furthermore, the pressure in the piping at the time of changeover is near atmospheric pressure and it takes a considerable interval of time for pump 43 to lower the pressure in the piping to the desired degree of vacuum even if the pump is of large capacity. Accordingly, the prior art system using the pump 43 alone to remove the residual gas, is not capable of quickly and smoothly evacuating the piping.